1. Field of the Invention
The present invention relates to pressure transient testing of producing hydrocarbon (oil and gas) reservoirs, and more particularly to measuring behind casing hydraulic conductivity between reservoir layers in connection with such pressure transient testing.
2. Description of the Related Art
Pressure transient tests are run on most wells in newly discovered hydrocarbon reservoirs. The results of such tests become a fundamental basis of assessing any future commercial producibility of the hydrocarbon (oil and gas) reservoirs, which includes important details such as economic forecasts based on predicted production rates, reserve assessments and plans for development of infrastructure to produce and transport the hydrocarbons to markets and consumers. During transient tests, both the production rates of fluids at surface and the pressures at downhole conditions are measured with time. Fluid samples are also collected and analyzed later in the laboratory for determining the engineering properties. The test data is analyzed combining with fluid properties to characterize the reservoirs. Such an analysis includes comparing the test data with the predicted or synthetic response of a conceptual model of the actual reservoir. It is important to utilize a realistic model of the reservoir for predicting its future commercial producibility. Hence an appropriate analytical model with two reservoir layers subject to flow behind casing has been incorporated in this invention.
Failure to account for the flow behind the casing may mislead regarding the source of produced fluids and may thus provide unrealistic results from transient tests, where stakes are high. Transient tests provide the characteristic parameters of the reservoir that are acquired under a dynamic condition, which resembles an actual producing condition of a well. That is why, forecasting, having utilized the test results, allows utilization of a realistic downhole condition of the hydrocarbon reservoir at the time of the test.
During pressure transient tests, reservoir permeability and/or mobility, formation damage parameter in terms of skin factors, reservoir pressure, reservoir size and shape, locations of geological features or boundaries are important parameters that are usually determined through such tests. To ascertain accuracy of the reservoir parameters, often individual reservoir layers located at different vertical depths are tested separately. The layers are usually separated by impermeable, non-reservoir strata whose thicknesses can vary from a few inches to few hundred feet.
In evaluating the productive capability of a subsurface reservoir layer, a test known as a transient pressure test is conducted for the layer under investigation. Sometimes the casing is perforated across all available permeable reservoir layers so that fluid from each layer can contribute to the total production through the wellbore, which is usually referred to as “commingled production”. For maximizing the hydrocarbon recovery from reservoirs under such a production arrangement, the operator of the oil or gas field needs to know the producibility of individual reservoir layers. Failure to gain this a priori knowledge may cause losing the hydrocarbons from some reservoir layers due to diversion of this fluid from one reservoir layer to another layer instead of flowing towards the wellbore during production or even shut-in. That is why the operator of the field would perform a pressure transient test on one reservoir layer at a time without interfering with any other adjacent reservoir layers. Interfering with the other reservoir layers is probable when any fluid from these layers flows to the layer under investigation or somehow gets produced through the wellbore. Such interference can cause an overestimation of the producibility of the layer under investigation. When the other layers adjacent to the layer under investigation have hydraulic connection to the wellbore through open perforations in the casing, these other layers are blocked from permitting flow from other layers into the casing by what are known as plugs. The annular space between the outside of the well (casing) and the drilled, rugged surface of fluid impermeable rock formation between the layers is filled with cement to, theoretically, block flow and provide zonal isolation between the layers. However, a complete block is sometimes not achieved and this contributes to a condition known as “behind casing flow” between layers.
When the zonal isolation (cement) behind the casing is compromised, the flow behind casing becomes prevalent. If the additional contribution from a layer that is not being tested is not recognized, the reserve estimates and the characterization of the tested reservoir layer can at times be erroneous and misleading.
So far as is known, the effect of possible presence of behind casing flow between layers in the data obtained during pressure transient tests has not been taken into account in the test data for formation layers of interest.